1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device. More particularly, the invention relates to imaging lenses which are built in image pickup devices mounted in increasingly compact and low-profile smartphones, mobile phones and PDAs (Personal Digital Assistants), information terminals such as game consoles and PCs, and further in consumer electronics added with a camera function and the like.
2. Description of the Related Art
In recent years, it is becoming popular to mount a camera function to numerous information terminals. Further, consumer electronics with camera are also appearing in the market, and for example, it becomes possible to view the situation of one's house in a timely fashion through the camera mounted in the devices even from outside, by using a smartphone to communicate with the consumer electronics. As explained above, it is expected that product development to improve convenience of consumers by integrating the camera function into the information terminals and consumer electronics will progress continuously in the future. Cameras to be mounted in such devices are naturally required to have high resolution corresponding to the increase in the number of pixels, and also required to be compact with a low height, have a bright lens system, and moreover correspond to wide field of view.
However, correction of aberrations is difficult at peripheral portion of an image, in obtaining a low-height, wide field of view, and bright imaging lens, and it is difficult to secure good optical performance for overall image. It is possible to solve these problems to a certain extent by adopting a glass material as a lens material. However, the glass material is not suitable for mass production, so that a problem arises in providing the lens at low cost in large quantities.
Conventionally, as an imaging lens aiming at wide field of view and high functionality, for example, the imaging lens described in JP-A-2010-271541 (Patent Document 1) and the imaging lens described in JP-A-2010-026434 (Patent Document 2) are known.
Patent Document 1 discloses an imaging lens which includes, arranged in order from an object side to an image side, a first lens with positive refractive power as a biconvex lens, a second lens with negative refractive power having a concave surface on an image-side surface, a third lens with positive refractive power as a meniscus lens having a convex surface on an image-side surface, and a fourth lens with negative refractive power having an aspheric shape on both surfaces and having a concave surface on an image-side surface near an optical axis.
Further, Patent Document 2 discloses an imaging lens which includes, in order from the object side, a positive first lens, a positive second lens, a negative third lens, a positive fourth lens, and a negative fifth lens, which aims at downsizing and favorable correction of various aberrations.
In the imaging lens described in Patent Document 1, total track length is in the order of 5.4 mm, so that reduction in height is relatively achieved. However, correction of aberrations is not sufficient, since it has four constituent lenses. Further, maximum field of view is in the order of 70 degrees to 75 degrees, so that a wide field of view is relatively obtained. However, it is difficult to correspond to recent request for wider field of view. Still further, F-value is in the order of 2.8, and it is difficult to correspond to recent request for brighter lens system.
In the imaging lens described in Patent Document 2, the total track length is in the order of 7.8 mm, and is longer than a length of a diagonal of an effective image plane of the image sensor, so that it is disadvantageous in reducing height. Further, it achieves a bright lens system which corrects various aberrations favorably with five constituent lenses, with an F-value in the order of 2.0 to 2.5. However, maximum field of view is in the order of 62 degrees, and it is difficult to correspond to wider field of view.
As stated above, in the conventional techniques, it is difficult to correspond to high-resolution, downsizing, reduction in height, and also satisfy the request for wider field of view.